Queries :: Table Of End Of Week Sales - Query Where There Is No Match

I have a table of end of week sales with ProductID, Volume_Sold, Year and WeekNo. I am about to create a historical table of RRP.

What is the best way to set this out so that I can query the two tables to that when I run a query over the two tables I get the correct price depending on the year and week number I am working with.

tblSales
Year int,
Week int,
CountryCode nvarchar (2),
StoreNo, nvarchar (35),
ProductId nvarchar (15),
Volume_Sold int;

My new table
"tblRRP"
Could contain
Year int,
Week int,
CountryCode nvarchar (2),
ProductId nvarchar (15),
RRP float;

The table is only appended to when the price changes. So some products may have a price increase 2 or 3 times a year others once every 18 months. And if the price changes any calculations need to allow for the 2 or 3 different RRPs the Product may have had during the queried period.

So that when I do year on year revenue calculations it works properly.